The estimation of seabed characteristics is made using multibeam and sidescan sounders. Typical of the sounding equipment used is that supplied by the Norwegian company Kongsberg Simrad. For example, the EM 1002 echo sounder records echoes over a 3.3° wide arc (measured fore to aft) and over a 150° sector (measured athwartships) at 95 kHz. For shallow water operations, a pulse of duration 0.2 ms is used and the amplitude of the backscattered echo is sampled at a suitable selected frequency in the range of about 5-15 kHz.
Conventionally, a multibeam sounder is mounted on the hull of a vessel or on a towfish travelling with a forward motion of up to 10 knots. The swath swept out by the sounder has a typical useful size of approximately 5 times the water depth. This wide swath is a considerable advantage as the resulting seabed maps provide greater detail and ensure that there are no uncharted shoals. Further, seabed maps can be produced more quickly, thus reducing ship survey time.
Frequently, multibeam echo sounders are connected to positioning equipment, heading and motion sensing instruments, as well as sound velocity sensors, in order to track the sounder's path and orientation over the seabed. The raw data collected are stored in digital form aboard ship, and receive subsequent processing on shore for “cleaning” (the elimination of unreasonable values) and enhancement.
The amplitude of echoes backscattered from the seabed varies with the type of material present on the seabed as well as factors such as the distance travelled by the pulse to the seabed (the “range”) and the angle at which the pulse is incident at the seabed (the “angle of incidence”). By measuring the amplitude of backscattered echoes, it is possible to estimate the composition of the seabed. The angle of incidence at the point from which a given detected sonar signal has been reflected is the angle made at the seabed by the arriving wavefront with a vector normal to the surface of the seabed.
The attenuation of the amplitude signal due to the range of the sound waves through seawater is caused both by the spherical spreading of the wave front as it expands out from its source and by absorption of sound energy in water. The absorption of sound in water depends on the temperature as well as the salinity. A technique, known as time-varying gain (“TVG”), is widely used to compensate for the attenuation caused by different ranges through seawater. This does no more than make a rough adjustment by multiplying the amplitude of the received signal by a factor depending on the range. TVG does not always correct precisely for both spreading loss and absorption of sound by water. Further, TVG does not adjust for the angle of incidence at all. To remove artifacts in the image that remain after applying an estimated TVG, it is popular to apply an adaptive gain based on recently recorded amplitudes. This is inappropriate for sediment classification as it makes the recorded backscatter from a sediment type dependent on adjacent areas and the survey direction.
In “A real Seabed Classification using Backscatter Angular Response at 95 kHz” [J. E. Hughes Clarke, B. W. Danforth and P. Valentine published in “High Frequency Acoustics in Shallow Water”, NATO SACLANTCEN, Lerici, Italy, July 1997], the authors describe a proposal for seabed classification based on the shape of the angular response curve (“AR”). The AR curve plots the amplitude of the backscatter signal against the angle of depression towards the seabed. The shape of the AR curve is considered in three domains, the length and slope of which are suggested as being determinative of the nature of the seabed. This research offers some insight into the classification problem but does not offer a means or methodology for determining the sediment type, nor any proposal for generating processed data that would adequately represent seabed classification by sediment type or otherwise.
In “Seabed Classification of Multibeam Sonar Images” [J. M. Preston, A. C. Christney, S. F. Bloomer and I. L. Beaudet published in the proceedings of the IEEE Ocean's 2001 Conference, Honolulu, November 2001] the authors, who include the inventors named in this present patent application, outline the context in which the present invention is described. They point out that a division of the seabed echo data and corresponding seabed sediment data into acoustic classes is useful because substrate characteristics for any given sediment class are relatively constant throughout such class and are distinct from those of other classes. This makes it possible to considerably reduce the amount of real sampling of the seabed that needs to be done in order to convert the acoustic classification into a classification by sediment type. The technique proposed by the authors relies on computing measures (“features”) that are used, alone or in combination, to infer an appropriate acoustic classification from the recorded data by conventional statistical techniques. Further detail of the techniques employed is provided below. As the processing performed makes use of the entire survey data set, the classification is conveniently done after the survey is complete. In general, this technique can be applied to backscatter data from any multibeam system, provided it is operated consistently during the survey.
Preston et al. in the foregoing paper suggest the computation of more than 130 features that can be used in a principal components analysis to determine the most effective combination of features to act as the predictor of sediment type. Examples suggested are mean, standard deviation, higher-order moments, histogram, quantile, power spectra and fractal dimension. As a final step, cluster analysis is suggested to optimally assign classifications to collected data.
In “HIPS: Hydrographic Information Processing System” [M. Gourley and D. Dodd, White Paper #21, CARIS, Fredericton, New Brunswick, 1998] the authors describe a Hydrographic Data Cleaning System (“HDCS”) that provides a set of interactive software tools for detecting and cleaning up raw swath data. A number of these tools rely on having a human being available to identify and correct (or remove) bad data through personal inspection of the data.
Among the published United States patent literature, there are two patents that have some relevance to the normalization technique of the invention. U.S. Pat. No. 5,493,619 describes a normalization technique for eliminating false detections in a mine detection process. U.S. Pat. No. 6,052,485 describes a method for automatically identifying clutter in a sonar image. Neither of these issued patents attempts to deal with the enhancement of sonar data for seabed classification.
There are a number of United States patents which disclose inventions of apparatus or methods related to multibeam sounders. U.S. Pat. Nos. 4,024,490, 5,177,710, 5,579,010 and 5,663,930 are representative and provide further details of the equipment and methods used in echo sounding.
In U.S. Pat. No. 6,549,853 Chakraborty et al. describe a technique for measuring seafloor roughness using a multibeam sounder. The method described there relies on fitting survey data to predictions from theoretical backscatter models combined with geometrical corrections and detailed knowledge of particular sonar systems. However, such a technique does not have the advantages of statistical approaches, which are intuitive and give useful results with a wide range of sonar systems that do not have to be calibrated.
The data used to estimate the character of the seabed are echoes recorded from sound pulses transmitted towards the seabed. Each echo measured is a time series of the amplitude of sound received from a point on the seabed by a detector and is a sequence of digital values sampled from the analog signal at equal time intervals. Each time series has a “dead” period corresponding to the round-trip of the pulse to the nearest survey point on the seabed, followed by a spike corresponding to the backscattered echo of the pulse from the seabed. The essence of the classification procedure is to compare features (statistical measures, particularly defined below) derived from the amplitudes measured at survey points within rectangular patches on the seabed and where the features are judged to be sufficiently “similar”, assign those patches to the same acoustic classification. Subsequently, the acoustic classifications are related to real attributes of the seabed by actual observation of the seabed at representative points.
However, the echo amplitudes are affected by a number of factors, each of which must be compensated for if a useful comparison is to be made between echoes received from different survey points:    (1) The best multibeam transmitters produce a wave approximately resembling a fan—having a broad arc shape transverse to the motion of the vessel (typically 150 degrees) and a narrow uniform angle along the vessel's path (typically 1-3 degrees). The pulse advances on the surface of a sphere and its intensity falls off according to an inverse square law.    (2) The transmitter does not produce a beam of uniform intensity across all angles. Typical transmitter elements: produce pulses varying considerably in amplitude according to the angle of depression.    (3) Seawater absorbs energy non-uniformly, typically dependent on both temperature and frequency. This causes a reduction in the amplitude of echoes detected.    (4) Because the backscatter is from a wide swath, and because the bottom is often irregular, the echo received varies substantially with the angle of incidence at the seabed.
In practice, items 2 and 3 above are dealt with by the manufacturers of multibeam sonar systems. In particular, such manufacturers are able, to some extent, to incorporate adjustments internally to compensate the echo signal for the non-uniformity of angular transmission and to generate frequencies in a narrow range.
Thus there remain two important factors, range (item 1) and angle of incidence (item 4), which require compensation before valid comparisons can be made between echoes received from different points on the seabed.